Although fiber-based cigarette filter elements are well known and have been used commercially for a number of years, the available choice of substrate components to form such filters has remained quite narrow, due to functional and cost factors, particularly the lack of suitability of many natural fibers for high speed production using a state-of-the art filter rod-making apparatus. In addition, functional demands on modern commercial cigarette filters tend to conflict with respect to various characteristics such as general and selective filtration efficiency, ease in handling, filter draw properties, and particularly filter element hardness.
For such reasons a substantial number of cigarette filter elements continue to be produced by use of old technology designed for handling cellulose acetate-based fiber.
Cellulose acetate fiber tow can be readily processed into cuttable filter rods using essentially unmodified state-of-the-art filter rod-making devices without serious jamming problems. This advantage is enjoyed despite present day need for a substantial number of additives such as, non-volatile organic plasticizers, inclusive of triacetin, diacetin, and citric acid, lubricants, flavors, medicines, selective filtering agents and the like. Such additives are most conveniently applied in the form of aqueous solutions onto opened fiber tow by conventional dipping, spraying, printing and the like. In the case of plasticizer additives, the resulting softened areas are capable of randomly adhering to adjacent fiber. This, in combination with crimp, is used to impart some degree of rigidity or hardness to the corresponding filter rod. Such treatment, in turn, permits further processing such as a rod-cutting step, to obtain desired filter element lengths.
The above-stated advantages of cellulose acetate fiber, however, are countered by serious disadvantages. For example the cellulose-based fiber tends to be relatively weak compared with synthetics such as polyolefin fiber. This characteristic seriously limits the amount of tension and crimp which can be imparted to filter-efficient low dpf fiber tow at production speeds, thereby requiring use of higher denier and reduced filter efficiency.
Synthetic fiber of filament components, particularly polyolefins such as polypropylene staple, on the other hand, offer a cheaper viable alternative since they can be drawn to a relatively small denier favoring a higher general filter efficiency without loss in tensile strength.
Synthetic fiber such as polyolefins, unfortunately, also have disadvantages. These generally relate to their chemically inert hydrophobic nature and the fact that a majority of known cigarette filter additives tend to be hydrophilic and difficult to apply and retain in functionally active mounts within a hydrophobic-type substrate or filter element.
Another substantial problem, unique to the cigarette filter art, concerns the difficulty in optimizing dimensional stability, hardness and draw of both cellulose acetate and synthetic fiber-containing cigarette filters without substantial cost increase due to required additional processing, higher substrate content and the like. In addition polyolefin-containing filter elements often demonstrate a significant correlation between draw (i.e. pressure drop) and hardness properties within cigarette filter elements. Furthermore, it is found difficult to avoid jamming of heavily impregnated substrate fed at high speed as garniture feed into a conventional filter rod-making apparatus, particularly when required additives must be dispersed in viscous carriers or vehicles.
It is an object of the present invention to more easily control the hardness properties of cigarette filter elements.
It is a further object to increase the hardness of cigarette filter elements without substantially increasing crimp, substrate concentration, or denier within the filter plug.